The present invention relates to combustors in gas turbine engines, and more particularly, to an improved combustor geometry for initiating an air film on a combustor liner of a gas turbine engine.
FIG. 1 is a simplified, partial cross-sectional illustration of a prior art dual annular combustor 10. Combustor 10 has an outer liner 12 and an inner liner 14. The outer liner 12 is connected to an outer dome 16 and the inner liner is connected to an inner dome 18. Outer liner 12 and inner liner 14 are provided with film cooling holes 20 which are drilled through the liners at an angle selected to establish a film of insulative cooling air over the inner surface of the liners. In one example, the holes 20 are angled at between about 20 to 30 degrees with respect to the liner surface and have a diameter of 20-40 mils. The film cooling holes 20 allow compressor discharge air indicated by arrows 22 to convectively cool the material surrounding the immediate area within the hole passageway. After the air exits from the hole, it further provides a barrier film protection 23 between the hot combustor gases in the interior of the combustion 10 and the liner surface 24 of both the inner and outer liners 14 and 12, respectively. This film is intended to prevent direct contact of the hot gases with the liner surface. FIG. 1A is an enlarged cross-sectional view of liner 12 more clearly showing the angled air holes 20 which provide the cooling air 22 for barrier film 23.
The dual annular combustor 10 of FIG. 1 extends circumferentially around an engine centerline (not shown) with a plurality of inner and outer swirlers 26 circumferentially spaced around the centerline. Swirlers 26 are alternatively referred to as carburator devices. The film cooling holes 20 are situated in such a manner as to provide a cooling air film 23 extending both downstream and circumferentially around the outer liner 12 and inner liner 14.
In order to maintain the uniformity of surface contact of barrier film cooling 23, an air film starter is needed. Typically, an air film starter, shown in FIG. 2, which is an enlarged view of the axially forward, outer corner of the combustor assembly of FIG. 1, has been formed by the relational geometry of the extreme forward end 30 of the outer liner 12 to the outer dome 16. The relational geometry of the extreme forward region 31 of the inner liner 14 to the inner dome 18 forms a film starter for the inner liner 14.
In FIG. 2, outer dome 16 has a lip region 28 which is located immediately radially inward from a forward end 30 of the outer liner 12. Holes 33 drilled within the lip region 28 of the dome 16 act as a film starter within a channel 32 in that compressor discharge air 22 is channeled through the channel 32 and proceeds to flow aftward along the interior surface 24 of the outer liner 12.
To ensure cooling performance, without film deterioration, a constant height and constant flow area must be maintained within the channel 32. However, due to manufacturing tolerances, substantial enough differences exist between the various domes which make up the annular combustor 10 that a constant height within the channel 32 is not uniformly maintained. This lack of uniformity in height and flow area passageway reduces the air film effectiveness. In that a film starter creates a flow in the air film which continues to flow aftward as additional air is injected into the air film flow path by the film cooling holes 20, the effectiveness and flow of this air film 23 along surface 24 is reduced because the concentricity and height uniformity of lip region 28 is not maintained. This will result in the air film downstream deterioration by not allowing the formation and continued buildup of a uniform air film along surface 24.
In the prior art, stack-up/concentricity effects and non-uniform height and area variation effects cause the amount of film air flow to be non-uniform such that the critical flow rate in local areas will fall below the requirements necessary to maintain a continuous film and film cooling build-up. This problem particularly manifests itself in a reduction in the downstream film cooling. If this reduction is large enough, it can cause the local liner temperature and temperature gradients to increase significantly to such a degree that liner cracking will result, and cause engine teardown for replacement.
Another problem encountered in the prior art which has a detrimental effect upon air film cooling starter is how the outer liner and inner liner are secured to a combustor casing or an inner support member of the gas turbine engine. If bolts or other securing means obstruct the air which is to be used as a film starter, the downstream cooling effects of the air will be reduced.
Thus, a need is seen for a combustor having a geometry which maximizes the cooling effects of air film starter discharge.